1. Field of the Invention
The present invention relates to an optical amplifier for collectively amplifying signal levels in a predetermined wavelength band.
2. Related Background Art
Due to social needs with the advent of advanced information society, research and development have been booming concerning large-capacity, high-speed optical communications and long-haul optical communications utilizing optical fiber transmission line networks. Here, wavelength division multiplexing (WDM) transmission systems are systems which carry out high-speed, large-capacity optical communications by transmitting WDM signals (a plurality of optical signals having different wavelengths) through an optical fiber line, and their development and introduction have been in progress in order to respond to the surge in demands for communications due to Internet and the like.
In such a WDM transmission system, optical amplifiers are typically installed as repeater stations at predetermined intervals in order to compensate for the transmission loss in WDM signals caused by long-haul transmissions. Each optical amplifier comprises an optical amplification section, such as an optical fiber doped with a fluorescent material excitable with pumping light, for amplifying optical signals inputted thereto; and a pumping light source for supplying the pumping light to the optical amplification section. A known example thereof is an erbium-doped fiber amplifier (EDFA).
Also, OXC (Optical Cross Connect) and OADM (Optical Add/Drop Multiplexer) are often employed in WDM transmission systems for their efficient operation and improved reliability. In recent years, in particular, research on large-capacity signal switching and acceleration in switching speed has been under way concerning such OXC, ODAM, and the like along with the increase in transmission capacity.
When an optical amplifier is employed in a transmission system equipped with optical devices having a signal switching function, such as the above-mentioned OXC and OADM, there is a possibility that a problem such as one that follows may occur. Namely, it is important for the optical amplifier to amplify WDM signals in a bundle with a constant gain (amplification factor). If the optical signals are partly switched at a high speed, so that the number of optical signals (number of channels) fed into the optical amplifier is changed abruptly, then transient gain fluctuations may be generated in the optical amplifier due to cross saturation. With respect to such a problem, it has been proposed to speed up an automatic gain control scheme for suppressing the gain fluctuation in the optical amplifier.
For example, as for the gain control in the optical amplifier, gain control techniques utilizing a part of noise light outputted from the optical amplification section that is called ASE (Amplified Spontaneous Emission) are described in Japanese Patent No. 2778438 (document 1) and H. Yoon et al., IEEE Photon. Technol. Lett., vol. 11. no. 3, pp. 316-318,1999 (document 2).
The inventors have studied the above-mentioned conventional techniques and, as a result, have found problems as follows. Namely, the optical amplifiers shown in the above-mentioned documents 1 and 2 have been problematic in that the power proportion of the component utilized for automatic power control (APC) in the noise light included in the amplified light outputted from the optical amplifier cannot fully be secured in terms of structure.
For example, a part of noise light tapped by an optical coupler is utilized as monitor light in the optical amplifier shown in document 1, whereas a part of noise light tapped by an arrayed waveguide grating (AWG) is utilized as monitor light in the optical amplifier shown in document 2, whereby none of the optical amplifiers can fully enhance the noise light power to be detected.
Conversely, it means that a considerable amount of noise light not utilized as monitor light is included in the amplified light outputted to the outside (fiber transmission line) from the optical amplifier. Thus, the conventional optical amplifiers have been problematic in that they lack a structure for fully eliminating noise light, thus degrading the noise characteristics, thereby failing to yield a sufficient transmission quality.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical amplifier which carries out automatic gain control by automatic power control utilizing noise light, and comprises a structure for enhancing the noise light power utilized for the automatic power control, thereby effectively preventing its transmission quality from degrating, and realizing control for constantly maintaining the amplification gain for each optical signal (constant gain control) with a higher speed and a higher accuracy.
For achieving the above-mentioned object, the optical amplifier according to the present invention comprises, at least, an optical amplification section such as an optical fiber doped with a rare earth element or the like, an optical circulator, an optical filter, a photodetector device, and a gain control system, wherein noise light separated from amplified light outputted from the optical amplification section is guided to a branch line different from a main transmission line through which amplified optical signals propagate, and then the noise light is utilized for constant gain control of the optical amplification section.
Specifically, the optical amplification section is an optical component for amplifying optical signals inputted together with pumping light, and includes, for example, an amplification optical fiber doped with a rare earth element such as Er. The pumping light source is an optical component such as LD (Laser Diode), and supplies pumping light to the optical amplification section. The optical circulator is an optical component for separating the noise light from the amplified light outputted from the optical amplification section, and has a first port for inputting the amplified light including the amplified optical signals and the noise light, a second port for outputting the amplified light from the first port to the branch line, and a third port for outputting, of the amplified light, the optical signals re-inputted by way of the second port toward the outside of the optical amplifier (toward the main transmission line). The optical filter is an optical component disposed on the second port of the optical circulator, passes therethrough the noise light of the amplified light outputted from the second port, and reflects the optical signals of the amplified light toward the second port. The photodetector device is an optical component such as PD (Photo Diode) for detecting the noise light power having passed through the optical filter. The gain control system controls the pumping power by monitoring the noise light power to control the gain in the optical amplification section.
The optical amplifier according to the present invention carries out automatic power control by utilizing most of the ASE noise light separated from the amplified light, thus enabling more efficient automatic gain control of optical amplification. Since most of the noise light is utilized by the above-mentioned configuration, the automatic gain control scheme, specifically, the circuit configuration of a gain control circuit or the like included in the above-mentioned gain control system, can be simplified, and also the automatic gain control can be accelerated. (It is important for the noise light utilized as monitor light to have a higher detection level in order to realize a higher speed in automatic gain control, since it is necessary for the PD to enhance its load resistance if the power of noise light to be detected is low, which inevitably hinders the control for the pumping light source and the like from following fluctuations in amplification gain.)
Further, in the optical amplifier according to the present invention, without part of noise light being tapped from a main transmission line such as an optical fiber transmission line network through which the amplified optical signals are to propagate, the amplified light is once guided by the optical circulator to a branch line different from the main transmission line, and then signal and noise light are separated from each other by use of the optical filter. Thus separated signals are reflected by the optical filter, so as to propagate through the main transmission line again, whereas most of the noise light pass through the optical filter, thereby reaching the photodetector device. Such a configuration enables most of the noise light occurring in the optical amplification section to be utilized for automatic power control. In this case, as the detected noise light power becomes higher, the change in gain of the optical amplification section can be measured with a higher speed and higher accuracy. Therefore, as compared with conventional optical amplifiers utilizing the branched part of noise light, the optical amplifier according to the present invention enables automatic gain control with a higher speed and higher accuracy.
The above-mentioned optical filter is characterized in that reflectivity in the optical filter is set at each wavelength of the amplified optical signals so as to equalize the wavelength dependence of amplification gain (flatten the signal level at each signal wavelength) in the optical amplification section. When the reflectivity in the optical filter is set as such, both of automatic gain control and gain equalization are realized.
The optical amplifier according to the present invention may further comprise an output-side variable optical attenuator, disposed between the third port of the optical circulator and the output end of the optical amplifier, for attenuating the optical signals outputted from the third port. Alternatively, the optical amplifier may further comprise an input-side variable optical attenuator, disposed between its input end and the optical amplification section, for attenuating the optical signals to be inputted to the optical amplification section. Installing a variable optical attenuator on the output side or input side of the optical amplifier as such further enables automatic control of the output level of each optical signal and the like.
Also, the optical amplifier according to the present invention may comprise a structure for changing the reflectivity in the optical filter for each wavelength of the amplified optical signals. This configuration enables gain equalization control following fluctuations in the operating state of the optical amplifier, and the like, thereby further improving the accuracy in automatic gain control. In this case, the optical amplifier further comprises an output control system for regulating the reflectivity in the optical filter and thereby controlling the output level of each optical signal to be outputted. Providing such an output control system enables various gain control methods in which the output control effected by the reflectivity control with respect to the optical filter and the output control by means of the variable optical attenuator.
Preferably, the above-mentioned optical filter includes fiber Bragg gratings each passing therethrough noise light in the amplified light and reflecting the associated amplified optical signals. However, optical filters other than the fiber Bragg gratings can also be used as long as they have similar reflecting functions. Further, the above-mentioned output control system comprises a structure for changing the reflectivity in the optical filter by applying a temperature change or a distortion caused by deformation to one or more predetermined parts of the optical filter.
The optical amplifier according to the present invention may comprise an optical component for dividing a signal wavelength band into local bands at a predetermined frequency interval, and separating the divided local bands into a group including the optical signals and a group including the noise light, in place of the optical circulator and the optical filter disposed on the branch line. This optical element has a first port for inputting the amplified light from the optical amplification section including noise light together with the amplified optical signals, a second port for outputting the amplified optical signals separated from the amplified light inputted by way of the first port, and a third port for outputting the noise light separated from the amplified light inputted by way of the first port. Such a configuration also enables gain control with a high speed and a high accuracy. Preferably, the frequency interval for dividing the signal wavelength band is substantially xc2xd of the frequency interval of the optical signals inputted to the optical amplification section.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.